Block
diagram of FM transmitter and receiver and its explanation
FM transmitter
Frequency Modulation is the process in
which the frequency of the carrier signal is varied by the modulating signal
while the amplitude remains constant
Using Reactance modulator direct method
The FM
transmitter has three basic sections.
- The exciter section contains the carrier oscillator, reactance modulator and the buffer amplifier.
- The frequency multiplier section, which features several frequency multipliers.
- The power output section,
which includes a low-
level power amplifier, the final power amplifier, and the impedance matching network to properly load the power section with the antenna impedance.
The
essential function of each circuit in the FM transmitter may be described as
follows.
- The Exciter
- The function of the carrier
oscillator is to generate
a stable sine wave signal at the rest frequency, when no modulation is applied. It must be able to linearly change frequency when fully modulated, with no measurable change in amplitude. - The buffer amplifier acts
as a constant high-
impedance load on the oscillator to help stabilize the oscillator frequency. The buffer amplifier may have a small gain. - The modulator acts to
change the carrier oscillator
frequency by application of the message signal. The positive peak of the message signal generally lowers the oscillator's frequency to a point below the rest frequency, and the negative message peak raises the oscillator frequency to a value above the rest frequency. The greater the peak-to-peak message signal, the larger the oscillator deviation. - Frequency Multiplier
Frequency multipliers are tuned-input, tuned-output
RF amplifiers in which the output resonant circuit is tuned
to a multiple of the input frequency. Common frequency
multipliers are 2x, 3x and 4x multiplication. A 5x
RF amplifiers in which the output resonant circuit is tuned
to a multiple of the input frequency. Common frequency
multipliers are 2x, 3x and 4x multiplication. A 5x
Frequency multiplier is sometimes seen, but its
extreme low efficiency forbids widespread usage. Note that multiplication is by
whole numbers only. There can not a 1.5x multiplier, for instance.
- Power output section
The final power section develops the carrier power, to be transmitted and often
has a low-power amplifier driven the final power amplifier. The impedance
matching network is the same as for the AM transmitter and matches the antenna
impedance to the correct load on the final over amplifier.
FREQUENCY MULTIPLIER
A special
form of class C amplifier is the frequency. Multiplier. Any class C amplifier
is capable of performing frequency multiplication if the tuned circuit in the
collector resonates at some integer multiple of the input frequency.
For
example a frequency doubler can be constructed by simply connecting a parallel
tuned circuit in the collector of a class C amplifier that resonates at twice
the input frequency. When the collector current pulse occurs, it excites or
rings the tuned circuit at twice the input frequency. A current pulse flows for
every other cycle of the input.
A Tripler
circuit is constructed in the same way except that the tuned circuit resonates
at 3 times the input - frequency. In this way, the tuned circuit receives one
input pulse for every three cycles of oscillation it produces Multipliers can
be constructed to increase the input
frequency
by any integer factor up to approximately 10. As' the multiplication factor
gets higher, the power output of the multiplier decreases. For most practical
applications, the best result is obtained with multipliers of 2 and 3.
Another
way to look the operation of class C multipliers is .to .remember that the
non-sinusoidal current pulse is rich in harmonics. Each time the pulse occurs,
the second, third, fourth, fifth, and higher harmonics are generated. The
purpose of the tuned circuit in the collector is to act as a filter to select
the desired harmonics.
In many
applications a multiplication factor greater than that achievable with a single
multiplier stage is required. In such cases two or more multipliers are
cascaded to produce an overall multiplication of 6. In the second example,
three multipliers provide an overall multiplication of 30. The total
multiplication factor is simply the product of individual stage multiplication
factors.
Reactance Modulator
The
reactance modulator takes its name from the fact that the impedance of the
circuit acts as a reactance (capacitive or inductive) that is connected in
parallel with the resonant circuit of the Oscillator. The varicap can only
appear as a capacitance that becomes part of the frequency determining branch
of the oscillator circuit. However, other discrete devices can appear as a
capacitor or as an inductor to the oscillator, depending on how the circuit is
arranged. A colpitts oscillator uses a capacitive voltage divider as the
phase-reversing feedback path and would most likely tapped coil as the
phase-reversing element in the feedback loop and most commonly uses a modulator
that appears inductive
FM RECEIVER
RF section
Consists of a pre-selector and an amplifier
Pre-selector is a broad-tuned band pass filter with an
adjustable center frequency used to reject unwanted radio frequency and to
reduce the noise bandwidth.
RF amplifier determines the sensitivity of the
receiver and a predominant factor in determining the noise figure for the
receiver.
Mixer/converter section
Consists of a radio-frequency oscillator and a mixer.
Choice of oscillator depends on the stability and
accuracy desired.
Mixer is a nonlinear device to convert radio frequency
to intermediate frequencies (i.e. heterodyning process).
The shape of the
envelope, the bandwidth and the original information contained in the envelope
remains unchanged although the carrier and sideband frequencies are translated
from RF to IF.
IF section
Consists of a series of IF amplifiers and band pass
filters to achieve most of the receiver gain and selectivity.
The IF is always lower than the RF because it is
easier and less expensive to construct high-gain, stable amplifiers for low
frequency signals.
IF amplifiers are also less likely to oscillate than
their RF counterparts.
Detector section
To convert the IF signals back to the original source
information (demodulation).
Can be as simple as a single diode or as complex as a
PLL or balanced demodulator.
Audio amplifier section
Comprises several cascaded audio amplifiers and one or
more speakers
AGC (Automatic Gain Control)
Adjust the IF amplifier gain according to signal level
(to the average amplitude signal almost constant).
AGC is a system by means of which the overall gain of radio receiver
is varied automatically with the variations in the strength of received
signals, to maintain the output constant.
AGC circuit is used to adjust and stabilize the
frequency of local oscillator.
Types of AGC –No AGC, Simple AGC, Delayed AGC.
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